1. Field of the Invention
The present invention relates in general to a film bulk acoustic wave resonator and manufacturing method thereof.
2. Description of the Related Art
For radio frequency (RF) band, a dielectric resonator, a metal cavity resonator and a piezoelectric thin film resonator (FBAR) are used.
These resonators are superior in terms of a small insertion loss, a frequency characteristic or temperature stability.
However, they are too big to be implemented as a compact and light integrated circuit on a semiconductor substrate.
When compared with the dielectric resonator or the metal cavity resonator, the FBAR can be manufactured to be compact and can be implemented on a silicon substrate or a gallium arsenic (GaAs) substrate, and can have a smaller insertion loss.
A filter, which uses the dielectric resonator, the metal cavity resonator and the FBAR, is one of the core components necessary for a mobile communication system.
Technology for the filter manufacture is indispensable to implement a compact, light and low power-consuming mobile terminal.
The dielectric filter and a Surface Acoustic Wave (SAW) filter are most widely used for RF for mobile communication.
The dielectric filter is used as a 900 MHz filter for a mobile phone for a home use, and a 1.8-1.9 GHz duplex filter for PCS. It features a high dielectric constant, a low insertion loss, stability in a temperature variation, a vibration resistance and a shock resistance.
However, it is difficult to implement a compact dielectric filter into a Monolithic Microwave Integrated Circuit (MMIC).
The SAW filter is smaller than the dielectric filter and can easily process a signal, and has advantages of a simple circuit and easy massproduction. In addition, the SAW filter does not need to be adjusted.
However, it is not easy to manufacture the SAW filter operating at more than a super high frequency (5 GHz or higher) band due to manufacturing process restrictions.
The FBAR filter is differentiated from the above filters in that it is very light and thin, and can be easily mass-produced by means of a semiconductor process and combined with RF active elements without any limitations.
The FBAR filter is a thin film where a cavity is created by a piezoelectric characteristic after a piezoelectric material, such as ZnO or AIN, is deposited on the silicon (Si) or GaAs substrate in a RF sputtering method.
An FBAR fabrication process comprises a membrane type, a Bragg reflector type and an air gap type.
In the Bragg reflector type FBAR fabrication method illustrated in FIG. 1, a reflection layer 11 formed by a material having a big acoustic impedance difference and deposited every other layer, a lower electrode 12, a piezoelectric layer 13 and an upper electrode 14 are formed on the substrate in order, where acoustic wave having passed the piezoelectric layer 13 is prevented from being transmitted further to the substrate direction and is totally reflected from the reflection layer 11, resulting in creation of an effective resonance.
The Bragg reflector type FBAR fabrication method is disadvantageous in that it is difficult to form a reflection layer structure having an accurate thickness of four or more reflection layers for total reflection and it takes long time and cost to manufacture, although there are advantages in that it is structurally rigid and no stress due to bending is generated.
Meanwhile, the air gap-type FBAR uses an air gap instead of the reflection layer to separate the substrate from the resonance part, and is divided into several types according to the manufacturing method used. Different types of air gap-type FBAR elements are illustrated in FIGS. 2A, 2B and 2C.
The FBAR element in FIG. 2A is a bulk micro-machining FBAR fabricated in such a manner that a membrane 21 is formed by SiO2, for example, on a substrate 20, a cavity part 23 is defined by the anisotropic etching of a rear side of the substrate 20, and the acoustic resonator 22 is formed on the membrane 21. An FBAR element thus structured is not practical due to its very weak structure and a low recovery rate.
The FBAR element in FIG. 2B is a surface micro-machining FBAR fabricated as follows.
A sacrificial layer is formed on a substrate 30, and an insulation membrane 32 is formed on the sacrificial layer and the substrate 30. A first electrode 33, a piezoelectric layer 34 and a second electrode 35 are vapor-deposited in order, and finally, the sacrificial layer is removed to form an air gap 31.
More specifically, a via hole is formed to connect the exterior of the element to the sacrificial layer inside the element, and an etchant is injected through the via hole to remove the sacrificial layer. Consequently, the air gap 31 is formed.
Furthermore, in manufacturing the membrane, the sacrificial layer needs to be slanted, which causes a weak structure due to a high remaining stress of the membrane.
The FBAR element of FIG. 2C is fabricated in the following manner.
A cavity part 45 is defined by etching a substrate 40 using a photo-resist membrane, and a sacrificial layer is vapor-deposited on the cavity part 45.
A membrane 41, a first electrode 42, a piezoelectric layer 43, and a second electrode 44 are vapor-deposited in order on the sacrificial layer and the substrate 40. Then, an air gap 45 is formed by etching the sacrificial layer.
In the above manufacturing method, a wet etching and a dry etching are employed in forming the air gap 45.
In case of wet etching, it is hard to remove the etchant. Moreover, when the etchant is not completely removed, the element becomes weak due to continuous actions of the etchant, and the resonance frequency may be changed.
In case of dry etching, on the other hand, the etching is accomplished by a plasmatic gas. At this time, physical impact can be caused by an ion and a molecule, and the membrane 41 or the substrate 40 can be deteriorated by high temperature.